(1) Field of the Invention
The present invention relates generally to a process for extracting and refining vegetable oils and to the resultant product, and in particular to the production by a combination of mechanical extraction and physical refining of non-hydrogenated soybean oil having an acceptable frylife similar to that of partially hydrogenated soybean oil.
(2) Description of the Prior Art
Soybean oil production involves several steps that are necessary to render the soybean oil suitable for human consumption. These production steps may be broadly characterized as 1) soybean preparation, 2) oil extraction, and 3) oil refining. Soybean preparation generally includes the steps of cleaning, drying, cracking, and dehulling.
The great majority of commercial soybean oil production processes extract or separate the oil from the soybean meal by a process known as solvent extraction. In the solvent extraction process, the prepared beans are first flaked to provide a large surface area. A solvent, commonly hexane, is then pumped through the soybean flakes to dissolve the oil in the hexane, separating approximately 99.5% of the oil from the meal. The hexane is then separated from the oil and recycled.
The crude oil resulting from the solvent extraction must then be subjected to additional treatments, collectively called “refining”, to remove various materials in order for the oil to be suitable for consumption. These materials include hydratable and non-hydratable phospholipids, free fatty acids, and various color and flavor components. Crude soybean oil contains phosphorous compounds called hydratable phospholipids, and small amounts of calcium and magnesium that complex with a portion of the phospholipids to form non-hydratable phospholipids. Hydratable phospholipids are normally removed by a process known as “degumming”, in which the oil is agitated or otherwise intimately combined with water to precipitate gums from the oil. The gums are then removed by centrifugation.
These precipitated gums can be used as a feed additive, or evaporated to remove moisture. The end product, lecithin, has various end uses such as in food emulsifiers. The degummed oil is dried under vacuum to remove any water. Removal of non-hydratable phospholipids is considerably more difficult and expensive, requiring further chemical treatment, typically chemical refining, to break the chemical bonds between the calcium or magnesium ions and the phospholipids, followed with extensive bleaching of the oil.
In most processes, free fatty acids are removed from the oil by a process known as caustic refining, also called chemical or alkali refining, in which the oil is mixed with a caustic material, such as sodium or potassium hydroxide, which undergoes a saponification reaction with the acids, forming soaps that are then removed by centrifugation. Non-hydratable phospholipids are removed along with the free fatty acids. Chemical refining soybean oil is an expensive process, requiring a large investment in capital equipment. In addition, a significant quantity of the oil is captured by the soaps, adversely affecting oil yield. Also, the caustic refining process produces soapstock, which has little commercial value, and is difficult to dispose of without environmental problems.
Conventional refining processes also involve some bleaching of the soybean oil to remove color pigments that adversely affect the color of the oil. Finally, chemicals that add flavors to the oil are removed by a process known as “deodorization”, which is essentially a form of physical distilling, in which the oil is subjected to high temperatures under a vacuum for a short period of time, which is sufficient to remove the flavor-causing components, but insufficient to break down non-hydratable phospholipids.
A major use of soybean oil is in deep frying of foods, such as chicken, fish, french fries, etc., either in the production of pre-cooked packaged foods, or in the preparation of foods for on-premise or carry-out consumption in restaurants and other commercial establishments. In deep frying, a container or vat is filled with cooking oil that is heated to a frying temperature, normally around 350° to 375° F. The uncooked food is then immersed in the hot oil for a sufficient time to effect the desired cooking, and then removed for serving or packaging.
Some of the oil in the vat is lost during cooking due to absorption and evaporation. The oil is replenished by adding fresh oil to the oil remaining in the vat, and the oil is reused. This procedure is repeated until the oil becomes unusable, as indicated by darkening of the oil and the food cooked in the oil, and/or by the observance of an undesirable taste or appearance in the food being cooked.
Non-hydrogenated soybean oil produced by solvent extraction and caustic refining is unsuitable for use in commercial frying operations due to its limited frylife. Such oil has a maximum frylife of only about 4–5 fry cycles, a cycle being the frying of one batch of food. Replacement of the oil at this frequency is uneconomical. In order to extend the frylife of soybean oil to a commercially acceptable number of fry cycles, preferably at least about 30 fry cycles, refined soybean oil is normally at least partially hydrogenated. Hydrogenation of solvent extracted, caustic refined soybean oil reduces the percentage of C 18:3 acids. It is commonly believed that the presence of C 18:3 acids contribute to the rapid deterioration, and thereby limited frylife, of solvent extracted, caustic refined soybean oil.
Saturation or hydrogenation of C 18:3 acids, however, reduces the healthful properties of soybean oil. Therefore, attempts have also been made to reduce the C 18:3 or linolenic acid content of soybean oil by genetically modifying or selective breeding the soybean, thereby enabling the production of non-hydrogenated soybean oil having a C 18:3 content of significantly less than the 6–8% by weight C 18:3 acids, based on the total content of free fatty acids, found in conventional soybeans. U.S. Pat. No. 5,981,781 to Knowlton, issued Nov. 9, 1999, is an example of these generically modified soybeans. This approach, to date, has met with at most limited success.
Commonly assigned U.S. Pat. No. 6,511,690, issued Jan. 28, 2003 and U.S. Pat. No. 6,753,029, issued Jun. 22, 2004, (herein the “Tysinger et al. patents”), both patents incorporated herein in their entireties by reference, describe an improved refining process combining mechanical extraction with physical refining. As noted in the Tysinger et al. patents, mechanical oil extraction and physical refining were previously known separately, but had not been used in combination, and these resultant properties of the resultant oil had not been appreciated.
In the prior art mechanical oil separation process known as expelling, dehulled beans are extruded through a screw press to frictionally heat the beans and rupture the oil cells. Within the screw press, the beans are subjected to high pressures and frictionally-generated high temperatures for a short period. The crushed, oil-containing meal is then pressed to separate most of the oil from the meal. This process has been rarely used to process soybeans due to the fact that about 25% of the soybean oil is left in the meal.
Physical refining has been used for oils that are naturally low in non-hydratable phospholipids, such as lauric oils, particularly palm oil. In physical refining, the oil is vacuum distilled at high temperatures, e.g., from about 450° F. to about 500° F., to separate more volatile components from the oil. This process is used to remove various flavor components, and will also remove free fatty acids. However, the process alone is not viable for removing free fatty acids from oils such as soybean oil, which contains higher levels, i.e., more than 20 ppm based on elemental phosphorous content, of non-hydratable phospholipids. The high temperatures required for physical refining tend to break down the non-hydratable phospholipids that are present in the soybean oil, producing chemical compounds that cause an unacceptable flavor and color.
In the process described in the Tysinger et al. patents, soybean oil is mechanically separated from prepared soybeans by first rapidly heating the beans to a temperature of from about 300° F. to about 370° F., preferably from about 315° F. to about 335° F., followed by mechanically pressing the oil from the beans. Desirably, the beans are crushed during or after heating to assist in freeing the oil from the remainder of the soybeans, i.e., the meal. After degumming and bleaching, the soybean oil is physically refined to remove free fatty acids and flavor components by heating the oil in a distillation column to a temperature of from about 450° F. to about 500° F., and preferably for from about 460° F. to about 480° F., to distill off the free fatty acids and flavor materials.
Tysinger et al. teach that care should be exercised in heating the soybeans at temperatures above about 350° F., since the oil will tend to scorch, causing an off taste in the final product and a darker color, and that heating the oil to less than 300° F. will fail to destroy sufficient trypsin inhibitors in the meal. Tysinger et al. also note that the time during which the soybeans are heated is also important, and that heating of the beans to the desired temperature in at least 10 seconds has been found to achieve maximum rupture of the oil cells, and thus maximum extraction of oil from the soybeans, while heating of the beans for longer than about 60 seconds degrades the desirable characteristics of the oil.
The restrictions of the Tysinger et al. process with regard to the desired temperature and time ranges requires careful control of the process and restricts the type of equipment that can be used during mechanical extraction. It would be desirable to have a process that was not constrained by these limitations while still producing a mechanically extracted soybean oil that was suitable for physically refining, thereby achieving an improved soybean oil of the type produced in the earlier Tysinger et al. process.